Superconductor rotor cooling system

ABSTRACT

A system for cooling a superconductor device includes a cryocooler located in a stationary reference frame and a closed circulation system external to the cryocooler. The closed circulation system interfaces the stationary reference frame with a rotating reference frame in which the superconductor device is located. A method of cooling a superconductor device includes locating a cryocooler in a stationary reference frame, and transferring heat from a superconductor device located in a rotating reference frame to the cryocooler through a closed circulation system external to the cryocooler. The closed circulation system interfaces the stationary reference frame with the rotating reference frame.

RELATED APPLICATION

This application is a continuation and claims the benefit of priorityunder 35 USC 120 of U.S. application Ser. No. 09/140,154, filed Aug. 26,1998 now U.S. Pat. No. 6,376,943. The disclosure of the priorapplication is considered part of and is incorporated by reference inthe disclosure of this application.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.DE-FC02-93CH10580 awarded by the Department of Energy. The Governmenthas certain rights in this invention.

BACKGROUND OF THE INVENTION

People have been concerned with how to cool the rotating elements of asuperconductor magnet. High temperature superconductor magnets typicallyneed to be cooled to a temperature of about 20-77 K during use.

It is known to place a cryocooler in the rotating reference frame of themagnet to cool the magnet windings. It is also known to force circulatea fluid between a stationary refrigerator and a rotating field winding.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a system for cooling asuperconductor device includes a cryocooler located in a stationaryreference frame and a closed circulation system external to thecryocooler. The closed circulation system interfaces the stationaryreference frame with a rotating reference frame in which thesuperconductor device is located.

Embodiments of this aspect of the invention may include one or more ofthe following features.

The closed circulation system includes a heat transfer assembly locatedin the rotating reference frame. A heat transfer gap is defined betweenthe cryocooler and the heat transfer assembly. Heat is transferred fromthe superconductor device to the heat transfer gap by the heat transferassembly. A coolant, for example, helium, is located in the heattransfer gap.

In illustrated embodiments, the rotating heat transfer assembly includesa heat pipe having a first fluid path for directing a flow of liquidcoolant, for example, liquid neon, from a cold end to a warm end of theheat transfer assembly, and a second fluid path for directing a flow ofgas coolant, for example, neon gas, from the warm end to the cold end ofthe heat transfer assembly.

A warm end conduction block is mounted to the superconductor device andthe heat pipe. The warm end conduction block defines the warm end of theheat transfer assembly. A cold end conduction block is mounted to theheat pipe and defines the cold end of the heat transfer assembly. Thecold end conduction block includes a first plurality of fins and thecryocooler includes a second plurality of fins intermeshed with thefirst plurality of fins. The cold end conduction block fins arerotatable with respect to the cryocooler fins. Space between theintermeshed fins defines the heat transfer gap.

In particular embodiments, a cooldown path containing, for example,liquid nitrogen or liquid oxygen, is provided to cool the superconductordevice prior to rotation of the superconductor device.

The cryocooler can include a plurality of coldheads. A heat pipe extendsfrom the plurality of coldheads. The heat transfer gap is definedbetween the heat pipe and the heat transfer assembly.

In particular embodiments, a coldhead of the cryocooler is locatedwithin an insulated enclosure. A rotatable shaft of the superconductordevice extends into the enclosure. A cold end of the shaft includes acondenser having a first plurality of fins. The coldhead includes asecond plurality of fins intermeshed with the condenser fins. Thecondenser fins are rotatable with respect to the coldhead fins.

In an other embodiment, a stationary cryocooler is positioned within arotatable shaft of the superconductor device. The rotatable shaftdefines flow channels for liquid coolant. The cryocooler includes anextension and coolant in the closed circulation system condenses uponcontact with the extension. The extension is radially aligned with thesuperconductor coils of the superconductor device.

The closed circulation system includes a fluid path for deliveringliquid coolant from a surface of the cryocooler to the superconductordevice, and a second fluid path for returning coolant vapor from thesuperconductor device to the surface of the cryocooler.

According to another aspect of the invention, a superconductor rotorcooling system includes a cryocooler located in a stationary referenceframe and a heat transfer assembly located in a rotating referenceframe. A heat transfer gap defined between the cryocooler and the heattransfer assembly transfers heat from a superconductor device located inthe rotating reference frame to the heat transfer gap.

According to another aspect of the invention, a method of cooling asuperconductor device includes the steps of locating a cryocooler in astationary reference frame, and transferring heat from a superconductordevice located in a rotating reference frame to the cryocooler through aclosed circulation system external to the cryocooler. The closedcirculation system interfaces the stationary reference frame with therotating reference frame.

According to another aspect of the invention, a method of cooling asuperconductor device includes the steps of locating a cryocooler in astationary reference frame, locating a heat transfer assembly in arotating reference frame, and transferring heat from a superconductordevice located in the rotating reference frame through the heat transferassembly to a heat transfer gap defined between the cryocooler and theheat transfer assembly.

Among other advantages, the cooling system of the invention permits thecryocooler to remain stationary while eliminating the need for anextensive sealing system needed to flow coolant through an opencirculation system. The heat transfer gap provides an efficientstructure for transferring heat from the superconductor device to thecryocooler.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the invention will be apparentfrom the following description taken together with the drawings inwhich:

FIG. 1 is a cross-sectional side view of a superconductor rotor coolingsystem;

FIG. 2 is an end view of the cooling system, taken along lines 2—2 inFIG. 1;

FIG. 3 is a partially cut-away side view of a cryocooler of the coolingsystem of FIG. 1;

FIG. 4 is an end view of the cryocooler, taken along lines 4—4 in FIG.3;

FIG. 5 is a cross-sectional side view of an alternative embodiment of asuperconductor rotor cooling system;

FIG. 6 is an end view of the cooling system of FIG. 5, taken along lines6—6 in FIG. 1;

FIG. 7 is an end view of the cooling system of FIG. 5, taken along lines7—7 in FIG. 1;

FIG. 8 is a cross-sectional side view of an alternative embodiment of asuperconductor rotor cooling system;

FIG. 9 is an end view of the cooling system of FIG. 8, taken along lines9—9 in FIG. 1;

FIG. 10 is a cross-sectional side view of an alternative embodiment of asuperconductor rotor cooling system;

FIG. 11 is a cross-sectional side view of a heat pipe bayonet of thecooling system of FIG. 10;

FIG. 12 is a cross-sectional side view of an alternative embodiment of asuperconductor rotor cooling system; and

FIG. 13 is a cross-sectional side view of an alternative embodiment of asuperconductor rotor cooling system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a superconductor rotor cooling system 10 includes,for example, a Gifford-McMahon (GM) cryocooler 12 located in astationary reference frame for cooling a high temperature superconductorwinding 18 located in a rotating reference frame. Other cooling systems,for example, pulse tube or Stirling cryocoolers, could be used.Cryocooler 12 is located in a stationary reference frame rather than arotating reference frame due to undesirable high gravity heat transferseen internal to the cold head of the cryocooler when rotating.

A closed circulation system 11 of rotor cooling system 10 interfaces thetwo reference frames to transfer heat from a winding 18 ofsuperconductor rotor 22 to cryocooler 12. Coolant within circulationsystem 11 at no time enters the cryocooler but rather is cooled bycontact with an external surface of the cryocooler, described below.Heat transfer within the circulation system occurs by various means, forexample, conduction, convection, and mass transport. No external force,for example, pumping, is applied to the coolant.

Cryocooler 12 is positioned within a hollow shaft 20 of a rotor 22. Abracket 24 mounted to shaft 20 on bearings 26 supports cryocooler 12such that cryocooler 12 remains stationary while shaft 20 rotates. Arelative motion gap 30 is defined between cryocooler 12 and an innerwall 28 of shaft 20. A seal 32, for example, a gas-to-gas, rubbing, orferrofluidic seal, separates relative motion gap 30 from a region 34within bracket 24. Relative motion gap 30 can be accessed by a feed line36 which passes through bracket 24 and seal 32 to introduce a coolant,for example, helium or neon, into gap 30.

Circulation system 11 includes a heat transfer assembly 16 having aconduction cylinder 40, a heat pipe assembly 42, and a cooldown line 44.Relative motion gap 30 includes a heat transfer gap 46 defined between acopper extension 48 of cryocooler 12 and cylinder 40. As discussedbelow, cryocooler extension 48 and cylinder 40 include a series ofinterleaved fins 50, 52, respectively, which define heat transfer gap46. Coolant within heat transfer gap 46 is cooled by contact with fins50 of cryocooler extension 48.

When superconductor rotor 22 is in use, heat is generated by winding 18and other parasitic heat leaks, such as radiation, conduction throughstructural supports and heat leak through the current leads. Todissipate the heat, heat is transferred by conduction to an innercooling block 54. The heat is then transferred from cooling block 54 tocylinder 40 by heat pipe assembly 42. Cooling block 54, heat pipeassembly 42, and cylinder 40 are located in the rotating referenceframe. The heat reaches cryocooler 12 by convection through the coolantlocated in gap 46.

Referring also to FIG. 2, heat pipe assembly 42 is preferably agravity-based neon heat pipe and includes a central pipe 60, three outerpipes 62 equally spaced about central pipe 60, and connecting pipes 64,six in total, connected each end of outer pipes 62 to central pipe 60.When heat pipe assembly 42 rotates, the neon within the pipes flowsradially outward to outer pipes 62 and toward the warmer end at coolingblock 54. The warmed neon now in the form of a gas travels in centralpipe 60 toward the colder end at cylinder 40. Thus, the neon in heatpipe assembly 42 is heated to a gas by conduction at cooling block 54,and is cooled to a liquid by conduction at cylinder 40. This mass fluxtransfers the heat flow from cooling block 54 to cylinder 40. The liquidand vapor flow results in a pressure head. A liquid head is provided byliquid neon located in connecting pipes 64 to balance the pressure dropof the pressure head.

When heat pipe assembly 42 is not rotating, for example, during cooldownof superconducting rotor 22, heat pipe assembly 42 operates in a gravitybased mode. Flow is provided by the liquid head acted upon by gravity.Under these conditions, a 0.25 inch head has been calculated to besufficient to support a heat flux of 60 watts for tube dimensions givenbelow. With heat pipe assembly 42 charged to 900 psi with neon, at 27 Kit has been calculated that there is sufficient liquid to fill outerpipes 62.

To decrease cooldown time, liquid nitrogen can be delivered to coolingblock 54 to decrease the temperature of winding 18 from ambient to 77 K.The liquid nitrogen is introduced at entry port 70 of cooldown line 44.The liquid nitrogen flowing through cooldown line 44 is heated byconduction at cooling block 54, and the nitrogen vapor exits cooldownline 44 at exit port 72. A bayonet type vacuum probe 74 is preferablyinserted into entry port 70 during cooldown with liquid nitrogenintroduced into cooldown line 44 through vacuum probe 74.

Referring to FIGS. 3 and 4, fins 50 on cryocooler extension 48 arecircular and concentrically arranged. Corresponding fins 52 on cylinder40 are also circular and concentrically arranged such that fins 50, 52intermesh as shown in FIG. 1. With a gap 46 of about 0.03 inch, fins 50,52 act to limit the temperature drop across heat transfer gap 46 to afew degrees Kelvin by increasing the surface area for heat transfer andby enhancing mixing and therefore increasing the convective heattransfer coefficient of the coolant located within heat transfer gap 46.The enhanced mixing of the coolant is caused by the interaction ofstationary fins 50 and rotating fins 52 on the coolant located betweenfins 50, 52.

A resistive heater 90 (FIG. 3) is used to control the temperature rangeof the neon within heat pipe assembly 42. Temperature control isnecessary because the condensation and boiling of the neon at the coldand hot ends of the heat pipe assembly occur only over a smalltemperature range. If the coolant in heat transfer gap 46 is neon,heater 90 is used to prevent the temperature of the neon from droppingbelow 24-25 K where neon freezes.

Heat pipe assembly 42, cooling block 54, cylinder 40 and extension 48are preferably formed of copper. Region 80 surrounding heat transferassembly 16 and region 82 surrounding winding 18 are held under vacuum.Fins 50, 52 are, for example, about 6 inches long, and extension 48 hasan outer diameter of about 4 inches. Tube 60 has an inner diameter ofabout 0.75 inch, and tubes 62 have an inner diameter of about 0.1 inchand are radially located about tube 60 on a diameter of about 4 inches.

Other embodiments are within the scope of the following claims. Forexample, referring to FIG. 5, heat transfer assembly 16 can be replacedwith a circulation system which relies on condensation and masstransport for cooling winding 218. A single copper extension 248 extendsfrom a cryocooler 212. Coolant located within a vacuum enclosure 217transfers heat from winding 218 to cryocooler 212. The enclosure definesa closed circulation system with coolant being vaporized at winding 218and condensed at copper finger 248.

To dissipate the heat from winding 218, vapor flows from winding 218 andcontacts extension 248 where the vapor is cooled and condenses to aliquid. The liquid coolant drops off extension 248 under the force ofgravity. As shown in FIG. 6, the liquid coolant 213 flows toward thewarmer end at coils 218 and is vaporized. Referring also to FIG. 7,rotor 222 can include a flow ring 215 defining slots 221 which aid inchanneling the liquid coolant toward the warm end. During cooldown thewinding may be cooled the same way or supplemented by an additionalbayonet. During cooldown, two phase nitrogen could be the preferredfluid, while during operation a lower boiling point fluid might bepreferred for heat transfer.

Referring to FIGS. 8 and 9, copper extension 248 of cold head 212 can beradially aligned within coils 218. In the configuration of FIG. 5, axialmass transport convects heat to the cryocooler interface 248, which ismore conveniently located in the shaft 217; while in the configurationof FIG. 8, the coldhead and heat transfer surface 248 exted radiallyinside coild 218 avoiding the necessity for axial heat transport.Alternative embodiments for the shape of the cooling system are shown inFIGS. 6 and 9.

Referring to FIGS. 10 and 11, to increase the cooling capacity of thecryocooler such that a broad range of refrigeration requirements can bemet, multiple coldheads 110, for example, two or three coldheads, can bebundled in a cryocooler assembly 112. A heat pipe bayonet 114 connectscoldheads 110 to extension 48 or 248. Bayonet 114 is gravity-fed tosupply condensed neon down a center tube 116. A return jacket 118provides a path for vapor to return to the coldhead. A vacuum jacket 120surrounds return jacket 118.

Referring to FIG. 12, in another embodiment, a hollow rotor 322 includesa condenser section 323 locating in the rotating frame. The condensersection is positioned within a stationary, vacuum insulated enclosure327. A coldhead 311 of a cryocooler 312 is located within enclosure 327.Coolant, for example, hydrogen, neon or nitrogen, in enclosure 327 iscooled by cryocooler 312. Coolant, for example, neon, within rotor 322evaporates at the coils and flows through rotor 322 to condenser 323where it is condensed to a liquid. The coolant within enclosure 327 andwithin rotor 322 define a closed circulation system. Condenser section323 includes fins 325, and coldhead 311 of a cryocooler 312 can includefins 313 intermeshed with fins 325.

Referring to FIG. 13, a closed circulation system includes a vacuuminsulated pipe 415 defining a first channel 417 which delivers liquidcoolant from a surface 441 of a coldhead 411 of a cryocooler 412 torotor 422, and a second channel 419 which returns coolant vapor to thesurface of the coldhead 411. Coldhead 411 is located in a vacuuminsulated enclosure 413. The cryogen is condensed at the surface of thecoldhead.

In one embodiment, the heat exchanger can be connected to the coldheadto increase the cold surface area. The liquid coolant moves fromcoldhead 411 to rotor 422 by gravity. The liquid coolant moves from thestationary frame to the rotating frame at pipe opening 423. Gravity,centrifugal force and wicks can be used to transport the liquid coolantto the coils. The annulus 427 between the stationary pipe 415 and therotating rotor is sealed by a seal 429, preferably a non-contactferrofluidic seal. Coolant vapor returns through channel 419 to coldhead411 by cryopumping. An additional warm vapor return line 431 can beprovided. If return line 431 is vacuum insulated, line 431 can alsoreturn intermediate temperature coolant to provide additional cooling tothe various loads. After cooling the winding, a portion of the returningflow can be diverted to intercept the heat loads to the current leads aswell as the parasitic load. The portion used to cool the parasitic loadswill be returned at intermediate temperature. A second coldhead may beincluded in some emobdiments.

What is claimed is:
 1. A system for cooling a superconductor devicelocated in a rotating reference frame, the system comprising: acryocooler located in a stationary reference frame; and a closedcirculation system external to the cryocooler, the closed circulationsystem, interfacing the stationary reference frame with the rotatingreference frame, effecting flow of a coolant, in liquid form, bygravity, from a first end in the stationary reference frame, to a secondend in the rotating reference frame, the second end being in thermalassociation with the superconductor device, and effecting a return flowof the coolant in vapour form from the second end to the first end; theclosed circulation system including a stationary pipe extending from thefirst end to the second end to direct the liquid coolant from the firstend to the second end.
 2. The system of claim 1, wherein the stationarypipe comprises walls forming a liquid-coolant channel for directingcoolant, in liquid form, from the first end to the second end.
 3. Thesystem of claim 1, wherein the stationary pipe comprises walls forming avapour-coolant channel for directing coolant, in vapour form, from thesecond end to the first end.
 4. The system of claim 1, wherein thestationary pipe is disposed to collect liquid coolant condensed on thecryocooler.
 5. The system of claim 1, further comprising a walls formingan additional coolant channel for directing coolant, in vapour form,from the superconductor device to the cryocooler.
 6. A rotating machinecomprising: a superconductor device located in a rotating referenceframe; and a system as recited in claim
 1. 7. A system for cooling asuperconductor device, the system comprising: a cryocooler located in astationary reference frame, and a passive closed circulation systemexternal to the cryocooler interfacing the stationary reference framewith a rotating reference frame in which the superconductor device islocated, the passive closed circulation system having a first end, asecond end, and a stationary pipe extending from a first end to a secondend to direct the liquid coolant from the first end to the second end.8. A method of cooling a superconductor device, comprising the steps of:locating a cryocooler in a stationary reference frame, and transferringheat from a superconductor device located in a rotating reference frameto the cryocooler through a passive closed circulation system externalto the cryocooler interfacing the stationary reference frame with therotating reference frame, the closed circulation system having a firstend, a second end, and a stationary pipe extending from a first end to asecond end to direct the liquid coolant from the first end to the secondend.